You have a few mistakes in your code -
1) you openned DISKSPACE for writing yet you are reading from it.
2) you had typo/spelling mistakes in the code.
3) you should use qx or `` to capture output from df.

I have written a demo on how to format df output according to your format.

#!/usr/bin/perl -w

format DISKSPACE =
======================
|@<<<<<<<<<<<<<<<<<<<|
$FileSystem
|@<<<<<<<<<<<<<<<<<<<|
$kbytes
|@<<<<<<<<<<<<<<<<<<<|
$used
|@<<<<<<<<<<<<<<<<<<<|
$avail
|@<<<<<<<<<<<<<<<<<<<|
$capacity
|@<<<<<<<<<<<<<<<<<<<|
$Mounted_on
======================
.


sub df_total
{
  my @lines = split /\n+/, shift;
  open (DISKSPACE, ">spaceavail.txt") || die "Error opening file space
+avail.txt";
  for (@lines) {
    ($FileSystem, $kbytes, $used, $avail, $capacity, $Mounted_on) = sp
+lit (/\s+/);
    write (DISKSPACE);
  }
  close (DISKSPACE)
}


my $result = `df -k`;
df_total($result);
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